Recent data from our laboratories have shown that approximately 60% of the human milk fed and 95% of the formula fed infants had selenium intakes below the 10 to 40 Mug/day proposed for infants 0-6 months old by the National Research Council (NRC). These findings emphasize the need to evaluate selenium nutriture during lactation and the period of rapid growth and development in infants. The overall objective of the proposed research is to determine the influence of dietary selenium intakes on human milk selenium content, localization and bioavailability. To meet this overall objective human and animal studies are proposed to: a) characterize the chemical forms and distribution of selenium in human milk; b) determine the influence of supplemental dietary selenium on human milk; c) determine the relationship between blood and human milk selenium fractions; d) determine the relative efficacy of supplemental dietary selenium as sodium selenite, selenocysteine, selenomethionine and selenium-enriched yeast on human milk content and distribution; e) study the influence of maternal selenium supplementation during lactation on infant selenium status; f) determine the relative bioavailability of human milk selenium using an animal bioassay. Considerable effort will be devoted to characterizing and quantitating the various selenium compounds that may be found in human milk. The total quantity of selenium and the content in the various fractions in the milk will be correlated with parameters of blood selenium status of the mother. The isolation of the various selenium containing compounds will be by the use of various classic methods for the separation of proteins, peptides, and amino acids. In addition, high pressure liquid chromatography, ion chromatography, and mass spectrometry will be used to assess individual selenium containing compounds. Once the content and distribution of selenium containing compounds is achieved, studies will be conducted to determine the influence of form and quantity of dietary selenium on milk selenium content and distribution. Differences in the content and/or distribution will then be evaluated for their effect on the bioavailability using the rat bioassay plus selenium status evaluation of the nursing infant.